The invention relates to a spray powder-coating system defined in the preamble of claim 1.
Such a spray powder-coating system is known from the European patent document 0 636 420 A3. Therein a pressure regulator is configured both in a pneumatic conveyance line and in an supplemental air line. A computer plots the powder conveyance rates (m) on a first coordinate axis and the air conveyance rates (FV) on a second coordinate axis. Moreover and with respect to at least one given embodiment of the spray powder-coating system, the graph shows a curve of the optimal total air rate (GV) consisting of the conveyance air and any supplemental air. A setpoint (set m) of the setpoint powder feed rates can be set at an input 52 of this computer. Based on this setpoint of powder feed rates, the computer calculates, by means of the curve of total air rates, the associated value FV of the air feed rate. Moreover, based on the differential of total air rate and air feed rate, the computer also computes any required supplemental air rate (set ZV). The setpoint air feed rates (set FV) and any required additional setpoint of supplemental air rate (set ZV) is used by the computer to drive the conveyance-air pressure-regulator and that for the supplemental air. Such a spray powder-coating system however operates fairly accurately only when the actual values of the conveyance air and of the supplemental air are taken into account in said regulation procedure. The regulators keep the air pressure constant in their lines. But this feature leads to a constant rate of conveyed airxe2x80x94in other words a constant quantity of air per unit timexe2x80x94only when the flow impedance remains constant downstream of the particular regulator. If said impedance does change, the rate of conveyance air will also change. The values and curves in said plot are from observation or obtained empirically for a given powder conveyance system. If an air hose connecting the injector to a control unit should be kinked, or if different lengths of such air hoses were to be used, or if one injector were replaced by another with a different flow impedance, the rate of conveyance air, the rate of any supplemental air and/or the total rate of air would therefore automatically change.
These fluctuations of the conveyance air rate will arise even when the computer memory stores plots for several different spray powder-coating systems because even in such a case inevitably air hoses shall be kinked or exchanged in the course of daily operations, and/or injectors shall be exchanged for others exhibiting different flow impedances.
On the other hand, to attain good efficiency in spray powder coating and a functionally as well as sightly satisfactory powder coating surface, the powder must be conveyed at specific, constant flow. If the flow is too low, there will be danger of powder deposits in the powder hose. If the conveyance rate is too high, the powder particles will recoil from the object being coated. Appropriate powder conveyance speeds are in the approximate range of 10 m/s to 20 m/s. However, to keep the powder flow at a given desired value, or within a range of desired values, the corresponding air flow conveying the powder must be kept correspondingly constant.
Air dividers are known from U.S. Pat. No. 3,625,404 and German patent 44 09 493 that comprise a throttling valve in a pneumatic conveyance line and a throttling valve in an supplemental pneumatic line. The two throttling valves are mechanically interlinked. To the extent one of them is opened, the other shall be closed. Throttling valves offer the advantage over pressure regulators that, in relation to their adjusted cross-sectional aperture and hence their adjusted flow impedance, whey will not keep pressure constant but instead will keep constant the airflow through them. A simple control unit is enough to adjust the throttles. A control circuit measuring the actual values is not required. Accordingly throttle valves may be construed being volumetric valves. The xe2x80x9cvolumetric flowxe2x80x9dxe2x80x94herein denoting xe2x80x9cvolume per unit timexe2x80x9dxe2x80x94is substantially independent of changes in the flow impedance in the flow path downstream of the flow throttle as long as said impedance remains relative small with respect to the flow valve""s impedance. However as regards spray powder coating systems the flow impedances in the injector and in the powder hose connecting the injector to the system already are large enough to present a drawback of flow throttles: This drawback is that the adjusting motion of the throttle does not entail a proportional or linear adjustment of the volumetric air flow through the throttle aperture. As a result, when using the known tandem throttles, only the theoretical total air xe2x80x9cflowxe2x80x9d,xe2x80x94herein xe2x80x9cflowxe2x80x9d denoting xe2x80x9cquantity per unit timexe2x80x94namely air conveyance flow plus supplemental air flow, will be set but not attained in practice. In order to attain accurate values, curved surfaces for the walls of the throttle aperture would have to be empirically determined in complex and time-consuming manner to attain linearity between adjusting the throttle cross-section and the resultant changes air conveyance flows. Such shapes of the throttling aperture cross-section would have to be determined for each variation of the spray powder coating systems exhibiting different flow impedances and each variation in turn would entail using correspondingly designed throttles.
The objective of the invention is to create an accurately operating but economical system not resorting to a complex and costly system of the kind disclosed in the European patent document 0 636 420 A and furthermore free of the throttling inaccuracies of the systems described in U.S. Pat. No. 3,625,404 and German patent 44 09 493.
This problem is solved by the features of claim 1.
In the invention, the throttling valves are inter-linked not mechanically but by a calculating mechanism and in particular by an electronic computer. This computer stores in a most simple manner the typical empirical values of at least one embodiment of a spray coating system. The representative values of a plurality of such systems can be stored in a computer or the like or computer and can be easily retrieved in programmed manner for application to the coating operation.